Fuse Technology
Circuit Protection
Electrical distribution systems are often quite complicated. They
cannot be absolutely fail-safe. Circuits are subject to destructive
overcurrents. Harsh environments, general deterioration, accidental damage, damage from natural causes, excessive expansion, and/or overloading of the electrical distribution system are
factors which contribute to the occurrence of such overcurrents.
Reliable protective devices prevent or minimize costly damage to
transformers, conductors, motors, and the other many components and loads that make up the complete distribution system.
Reliable circuit protection is essential to avoid the severe monetary losses which can result from power blackouts and prolonged
downtime of facilities. It is the need for reliable protection, safety,
and freedom from fire hazards that has made the fuse a widely
used protective device.
Overcurrents
An overcurrent is either an overload current or a short-circuit current. The overload current is an excessive current relative to normal operating current, but one which is confined to the normal
conductive paths provided by the conductors and other components and loads of the distribution system. As the name implies,
a short-circuit current is one which flows outside the normal conducting paths.
Overloads
Overloads are most often between one and six times the normal
current level. Usually, they are caused by harmless temporary
surge currents that occur when motors are started-up or transformers are energized. Such overload currents, or transients, are
normal occurrences. Since they are of brief duration, any temperature rise is trivial and has no harmful effect on the circuit
components. (It is important that protective devices do not react
to them.)
Continuous overloads can result from defective motors (such as
worn motor bearings), overloaded equipment, or too many loads
on one circuit. Such sustained overloads are destructive and
must be cut off by protective devices before they damage the
distribution system or system loads. However, since they are of
relatively low magnitude compared to short-circuit currents,
removal of the overload current within minutes will generally prevent equipment damage. A sustained overload current results in
overheating of conductors and other components and will cause
deterioration of insulation, which may eventually result in severe
damage and short-circuits if not interrupted.
Short-Circuits
Whereas overload currents occur at rather modest levels, the
short-circuit or fault current can be many hundred times larger
than the normal operating current. A high level fault may be
50,000A (or larger). If not cut off within a matter of a few thousandths of a second, damage and destruction can become
Bussmann®
rampant—there can be severe insulation damage, melting of
conductors, vaporization of metal, ionization of gases, arcing,
and fires. Simultaneously, high level short-circuit currents can
develop huge magnetic-field stresses. The magnetic forces
between bus bars and other conductors can be many hundreds
of pounds per linear foot; even heavy bracing may not be adequate to keep them from being warped or distorted beyond
repair.
Fuses
The fuse is a reliable overcurrent protective device. A “fusible” link
or links encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse.
Electrical resistance of the link is so low that it simply acts as a
conductor. However, when destructive currents occur, the link
very quickly melts and opens the circuit to protect conductors
and other circuit components and loads. Fuse characteristics are
stable. Fuses do not require periodic maintenance or testing.
Fuses have three unique performance characteristics:
1. Modern fuses have an extremely “high interrupting
rating”—can withstand very high fault currents without
rupturing.
2. Properly applied, fuses prevent “blackouts.” Only the
fuse nearest a fault opens without upstream fuses
(feeders or mains) being affected—fuses thus provide
“selective coordination.” (These terms are precisely
defined in subsequent pages.)
3. Fuses provide optimum component protection by
keeping fault currents to a low value…They are said to
be “current limiting.”
Voltage Rating
The voltage rating of a fuse must be at least equal to or greater
than the circuit voltage. It can be higher but never lower. For
instance, a 600V fuse can be used in a 208V circuit.
The voltage rating of a fuse is a function of its capability to
open a circuit under an overcurrent condition. Specifically,
the voltage rating determines the ability of the fuse to suppress
the internal arcing that occurs after a fuse link melts and an arc
is produced. If a fuse is used with a voltage rating lower than the
circuit voltage, arc suppression will be impaired and, under some
fault current conditions, the fuse may not clear the overcurrent
safely. Special consideration is necessary for semiconductor fuse
and medium voltage fuse applications, where a fuse of a certain
voltage rating is used on a lower voltage circuit.
Ampere Rating
Every fuse has a specific ampere rating. In selecting the ampere
rating of a fuse, consideration must be given to the type of load
and code requirements. The ampere rating of a fuse normally
should not exceed the current carrying capacity of the circuit. For
210
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Fuse Technology
instance, if a conductor is rated to carry 20A, a 20A fuse is the
largest that should be used. However, there are some specific
circumstances in which the ampere rating is permitted to be
greater than the current carrying capacity of the circuit. A typical
example is the motor circuit; dual-element fuses generally are
permitted to be sized up to 175% and non-time-delay fuses up
to 300% of the motor full-load amperes. As a rule, the ampere
rating of a fuse and switch combination should be selected at
125% of the continuous load current (this usually
corresponds to the circuit capacity, which is also selected at
125% of the load current). There are exceptions, such as when
the fuse-switch combination is approved for continuous operation at 100% of its rating.
Interrupting Rating
A protective device must be able to withstand the destructive
energy of short-circuit currents. If a fault current exceeds the
capability of the protective device, the device may actually rupture, causing additional damage. Thus, it is important when
applying a fuse or circuit breaker to use one which can sustain
the largest potential short-circuit currents. The rating which
defines the capacity of a protective device to maintain its integrity when reacting to fault currents is termed its “interrupting
rating”. The interrupting rating of most branch-circuit, molded
case, circuit breakers typically used in residential service entrance
panels is 10,000A. (Please note that a molded case
circuit breaker’s interrupting capacity will typically be lower than
its interrupting rating.) Larger, more expensive circuit breakers
may have interrupting ratings of 14,000A or higher. In contrast,
most modern, current-limiting fuses have an interrupting rating of
200,000 or 300,000A and are commonly used to protect the
lower rated circuit breakers. The National Electrical Code,
Section 110-9, requires equipment intended to break current at
fault levels to have an interrupting rating sufficient for the current
that must be interrupted.
Selective Coordination – Prevention of Blackouts
The coordination of protective devices prevents system power
outages or blackouts caused by overcurrent conditions. When
only the protective device nearest a faulted circuit opens and
larger upstream fuses remain closed, the protective devices are
“selectively” coordinated (they discriminate). The word “selective”
is used to denote total coordination…isolation of a faulted circuit
by the opening of only the localized protective device.
KRP-C
1200SP
LPS-RK
600SP
LPS-RK
200SP
2:1 (or more)
2:1 (or more)
This diagram shows the minimum ratios of ampere ratings of LOW-PEAK
YELLOW fuses that are required to provide “selective coordination”
(discrimination) of upstream and downstream fuses.
Unlike electro-mechanical inertial devices (circuit breakers), it is a
simple matter to selectively coordinate fuses of modern design.
By maintaining a minimum ratio of fuse-ampere ratings between
an upstream and downstream fuse, selective coordination is
assured.
Current Limitation – Component Protection
Areas within waveform
loops represent destructive
energy impressed upon
circuit components
Normal
load current
Initiation of
short-circuit
current
Circuit breaker trips
and opens short-circuit
in about 1 cycle
A non-current-limiting protective device, by permitting a shortcircuit current to build up to its full value, can let an immense amount of
destructive short-circuit heat energy through before opening the circuit.
Fuse opens and clears
short-circuit in less
than cycle
A current-limiting fuse has such a high speed of response that it cuts off a
short-circuit long before it can build up to its full peak value.
If a protective device cuts off a short-circuit current in less than
one-quarter cycle, before it reaches its total available (and highly
destructive) value, the device is a “current-limiting” device. Most
modern fuses are current-limiting. They restrict fault currents to
such low values that a high degree of protection is given to circuit
components against even very high short-circuit currents. They
permit breakers with lower interrupting ratings to be used. They
can reduce bracing of bus structures. They minimize the need of
other components to have high short-circuit current “withstand”
ratings. If not limited, short-circuit currents can reach levels of
30,000 or 40,000A or higher in the first half cycle (.008 seconds,
60 hz) after the start of a short-circuit. The heat that
can be produced in circuit components by the immense energy
of short-circuit currents can cause severe insulation damage or
even explosion. At the same time, huge magnetic forces
developed between conductors can crack insulators and distort
and destroy bracing structures. Thus, it is important that
a protective device limit fault currents before they reach their
full potential level.
211
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Fuse Technology
Bussmann®
Operating Principles of Bussmann® Fuses
The principles of operation of the modern, current-limiting Buss
fuses are covered in the following paragraphs.
Non-Time-Delay Fuses
The basic component of a fuse is the link. Depending upon the
ampere rating of the fuse, the single-element fuse may have one
or more links. They are electrically connected to the end blades (or
ferrules) (see Figure 1) and enclosed in a tube or cartridge surrounded by an arc quenching filler material. BUSS® LIMITRON®
and T-TRON® fuses are both single-element fuses.
Under normal operation, when the fuse is operating at or near its
ampere rating, it simply functions as a conductor. However, as
illustrated in Figure 2, if an overload current occurs and persists for
more than a short interval of time, the temperature of the link eventually reaches a level which causes a restricted segment of the link
to melt. As a result, a gap is formed and an electric arc established. However, as the arc causes the link metal to burn back, the
gap becomes progressively larger. Electrical resistance of the arc
eventually reaches such a high level that the arc cannot be sustained and is extinguished. The fuse will have then completely cut
off all current flow in the circuit. Suppression or quenching of the
arc is accelerated by the filler material. (See Figure 3.)
Single-element fuses of present day design have a very high
speed of response to overcurrents. They provide excellent shortcircuit component protection. However, temporary, harmless
overloads or surge currents may cause nuisance openings unless
these fuses are oversized. They are best used, therefore, in circuits not subject to heavy transient surge currents and the temporary over-load of circuits with inductive loads such as motors,
transformers, solenoids, etc. Because single-element, fast-acting
fuses such as LIMITRON and T-TRON fuses have a high speed of
response to short-circuit currents, they are particularly suited for
the protection of circuit breakers with low interrupting ratings.
Whereas an overload current normally falls between one and
six times normal current, short-circuit currents are quite high.
The fuse may be subjected to short-circuit currents of 30,000
or 40,000A or higher. Response of current limiting fuses to such
currents is extremely fast. The restricted sections of the fuse link
will simultaneously melt (within a matter of two or three-thousandths of a second in the event of a high-level fault current).
The high total resistance of the multiple arcs, together with the
quenching effects of the filler particles, results in rapid arc suppression and clearing of the circuit. (Refer to Figures 4 & 5) Shortcircuit current is cut off in less than a half-cycle, long before the
short-circuit current can reach its full value (fuse operating in its
current limiting range).
Figure 1. Cutaway view of typical single-element fuse.
Figure 2. Under sustained overload, a section of the link melts and an
arc is established.
Figure 3. The “open” single-element fuse after opening a circuit
overload.
Figure 4. When subjected to a short-circuit current, several
sections of the fuse link melt almost instantly.
Figure 5. The “open” single-element fuse after opening a short circuit.
212
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Fuse Technology
Bussmann Dual-Element Fuses
®
There are many advantages to using these fuses. Unlike single-element fuses, the Bussmann® dual-element, time-delay fuses can be sized closer to provide both high performance shortcircuit protection and reliable overload protection in circuits subject to temporary overloads and surge currents. For ac motor loads, a single-element fuse may need to be sized at 300% of an
a.c. motor current in order to hold the starting current. However, dual-element, time delay fuses can be sized much closer to motor loads. For instance, it is generally possible to size FUSETRON®
Dual-Element Fuses, FRS-R and FRN-R and LOW-PEAK® Dual-Element Fuses, LPS-RK_SP and LPN-RK_SP, at 125% and 130% of motor full load current, respectively. Generally, the LOWTM
PEAK® Dual-Element Fuses, LPJ_SP, and CUBEFuse , TCF, can be sized at 150% of motor full load amperes. This closer fuse sizing may provide many advantages such as: (1)
smaller fuse and block, holder or disconnect ampere rating and physical size, (2) lower cost due to lower ampere rated devices and possibly smaller required panel space, (3) better
short-circuit protection – less short-circuit current let-through energy, and (4) potential reduction in the arc flash hazard.
Insulated end-caps to help prevent
accidental contact with live parts.
Filler material
Figure 6. This is the LPS-RK100SP, a 100A, 600V LOW-PEAK®, Class RK1, Dual-Element Fuse that has excellent time-delay, excellent current-limitation and a 300,000A interrupting rating. Artistic liberty is taken to illustrate the internal portion of this fuse. The real fuse has a non-transparent tube and special small granular, arc-quenching material completely filling the
internal space.
Small volume of metal to vaporize
Short-circuit element
Overload element
Figure 7. The true dual-element fuse has distinct and separate overload
element and short-circuit element.
Figure 9. Short-circuit operation: Modern fuses are designed with minimum metal in the
restricted portions which greatly enhance their ability to have excellent current-limiting
characteristics – minimizing the short circuit let-through current. A short-circuit
current causes the restricted portions of the short-circuit element to vaporize and arcing
commences. The arcs burn back the element at the points of the arcing. Longer arcs result,
which assist in reducing the current. Also, the special arc quenching filler material contributes to extinguishing the arcing current. Modern fuses have many restricted portions,
swhich results in many small arclets – all working together to force the current to zero.
Before
Filler quenches the arcs
Spring
After
Figure 8. Overload operation: Under sustained overload conditions, the trigger spring
fractures the calibrated fusing alloy and releases the “connector”. The insets represent a
model of the overload element before and after. The calibrated fusing alloy connecting the
short-circuit element to the overload element fractures at a specific temperature due to a
persistant overload current. The coiled spring pushes the connector from the short-circuit
element and the circuit is interrupted.
Figure 10. Short-circuit operation: The special small granular, arc-quenching material
plays an important part in the interruption process. The filler assists in quenching the arcs;
the filler material absorbs the thermal energy of the arcs, fuses together and creates an
insulating barrier. This process helps in forcing the current to zero. Modern current-limiting fuses, under short-circuit conditions, can force the current to zero and complete the
interruption within a few thousandths of a second.
When the short-circuit current is in the current-limiting range of a fuse, it is not possible for the full available short-circuit current to flow through the fuse – it’s a matter of physics. The small
restricted portions of the short-circuit element quickly vaporize and the filler material assists in forcing the current to zero. The fuse is able to “limit” the short-circuit current.
Overcurrent protection must be reliable and sure. Whether it is the first day of the electrical system or thirty or more years later, it is important that overcurrent protective devices perform under
overload or short-circuit conditions as intended. Modern current-limiting fuses operate by very simple, reliable principles.
213
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Fuse Technology
This particular plot reflects the characteristics of a 200A, 250V,
LOW-PEAK YELLOW dual-element fuse. Note that at the 1,000A
overload level, the time interval which is required for the fuse to
open is 10 seconds. Yet, at approximately the 2,200A overcurrent level, the opening (melt) time of a fuse is only 0.01 seconds.
It is apparent that the time intervals become shorter as the overcurrent levels become larger. This relationship is termed an
inverse time-to-current characteristic. Time-current curves are
published or are available on most commonly used fuses showing “minimum melt,” “average melt” and/or “total clear” characteristics. Although upstream and downstream fuses are easily
coordinated by adhering to simple ampere ratios, these time-current curves permit close or critical analysis of coordination.
Better Motor Protection in Elevated Ambients
The derating of dual-element fuses based on increased ambient
temperatures closely parallels the derating curve of motors in elevated ambient. This unique feature allows for optimum protection
of motors, even in high temperatures.
Affect of ambient temperature on operating characteristics of FUSETRON
400
300
200
LOW-PEAK YELLOW
LPN-RK200 SP (RK1)
100
80
60
40
30
20
10
8
6
TIME IN SECONDS
Fuse Time-Current Curves
When a low level overcurrent occurs, a long interval of time will
be required for a fuse to open (melt) and clear the fault. On the
other hand, if the overcurrent is large, the fuse will open very
quickly. The opening time is a function of the magnitude of the
level of overcurrent. Overcurrent levels and the corresponding
intervals of opening times are logarithmically plotted in graph
form as shown to the right. Levels of overcurrent are scaled on
the horizontal axis; time intervals on the vertical axis. The curve is
thus called a “time-current” curve.
4
3
2
1
.8
.6
.4
.3
.2
150
140
PERCENT OF RATING OR
OPENING TIME
130
.1
.08
.06
Affect on Carrying
Capacity Rating
120
110
100
.04
90
80
70
.03
Affect on
Opening Time
.02
60
50
AMBIENT
176°F
(80°C)
212°F
(100°C)
6,000
8,000
10,000
140°F
(60°C)
3,000
4,000
104°F
(40°C)
2,000
68°F
(20°C)
600
800
1,000
–32°F
(0°C)
300
400
–76°F
–40°F
–4°F
(–60°C) (–40°C) (–20°C)
200
30
100
.01
40
CURRENT IN AMPERES
and LOW-PEAK YELLOW Dual-Element Fuses.
214
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Fuse Technology
Bussmann®
Better Protection Against Motor Single Phasing
When secondary single-phasing occurs, the current in the remaining phases increases to approximately 200% rated full load
current. (Theoretically 173%, but change in efficiency and power
factor make it about 200%.) When primary single-phasing
occurs, unbalanced voltages occur on the motor circuit causing
currents to rise to 115%, and 230% of normal running currents
in delta-wye systems.
Dual-element fuses sized for motor running overload protection
will help to protect motors against the possible damages of
single-phasing.
Classes of Fuses
Safety is the industry mandate. However, proper selection, overall
functional performance and reliability of a product are factors
which are not within the basic scope of listing agency activities.
In order to develop its safety test procedures, listing agencies
develop basic performance and physical specifications or standards for a product. In the case of fuses, these standards have
culminated in the establishment of distinct classes of low-voltage
(600V or less) fuses; classes RK1, RK5, G, L, T, J, H and CC being
the more important.
The fact that a particular type of fuse has, for instance, a classification of RK1, does not signify that it has the identical function or
performance characteristics as other RK1 fuses. In fact, the LIMITRON® non-time-delay fuse and the LOW-PEAK YELLOW™
dual-element, time-delay fuse are both classified as RK1.
Substantial differences in these two RK1 fuses usually requires
considerable difference in sizing. Dimensional specifications of
each class of fuse does serve as a uniform standard.
In the above illustration, a grooved ring in one ferrule provides the
rejection feature of the Class R fuse in contrast to the lower
interrupting rating, non-rejection type.
Branch-Circuit Listed Fuses
Branch-circuit listed fuses are designed to prevent the installation
of fuses that cannot provide a comparable level of protection to
equipment.
The characteristics of Branch-circuit fuses are:
1. They must have a minimum interrupting rating of 10,000A
2. They must have a minimum voltage rating of 125V.
3. They must be size rejecting such that a fuse of a lower
voltage rating cannot be installed in the circuit.
4. They must be size rejecting such that a fuse with a current
rating higher than the fuseholder rating cannot be installed.
Class R Fuses
Class R (“R” for rejection) fuses are high performance, ⁄Ω¡º to 600A
units, 250V and 600V, having a high degree of current limitation
and a short-circuit interrupting rating of up to 300,000A (rms symmetrical). BUSS Class R's include Classes RK1 LOW-PEAK YELLOW™ and LIMITRON® fuses, and RK5 FUSETRON® fuses. They
have replaced BUSS K1 LOW-PEAK and LIMITRON fuses and
K5 FUSETRON fuses. These fuses are identical, with the exception of a modification in the mounting configuration called a “rejection feature”. This feature permits Class R fuses to be mounted in
rejection type fuseclips. “R” type fuseclips prevent older type
Class H, ONE-TIME and RENEWABLE fuses from being installed.
The use of Class R fuseholders is thus an important safeguard.
The application of Class R fuses in such equipment as disconnect
switches permits the equipment to have a high interrupting rating.
NEC Articles 110-9 and 230-65 require that protective devices
have adequate capacity to interrupt short-circuit currents. Article
240-60(b) requires fuseholders for current-limiting fuses to reject
non-current-limiting type fuses.
215
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Transformers
(N.E.C. 450-3)
600V
Nominal
or Less
Over
600V
Nominal
Primary
and
Secondary
Protection
Primary
Protection
Only
Un-Supervised
Installations
Supervised
Installations
With
Thermal
Overload
Protection
Without
Thermal
Overload
Protection
Based on 1996 N.E.C.®
Transformer
Impedance of
More Than 6%
But Less
Than 10%
Transformer
Impedance of
6% or Less
Rated secondary current 9
amps or greater.
Rated secondary current less
than 9 amps.
Rated secondary current 9
amps or greater.
F
E
D
F
E
D
C
C
Primary and secondary
fuses at 125% of
primary and secondary
F.L.A. or next size
larger.
B
Rated secondary current 9
amps or greater.
Rated secondary current less
than 9 amps.
B
A
Rated secondary current less
than 9 amps.
125% or next size larger
125% or next size larger
Rated primary current
greater than or equal to
2 amps but less than 9 amps.
Rated primary current greater
than or equal to 9 amps.
125% or next size larger
(LPN-RK_SP, LPS-RK_SP,
FRN-R, FRS-R)
A
Secondary 600V or
Below
250%
250%
600%
600%
400%
400%
167% or next size smaller.
125% or next size larger.*
167% or next size smaller.
125% or next size larger.*
167% or next size smaller.
125% or next size larger.*
*When 125% of F.L.A. corresponds to
a standard rating, the next larger size
is not permitted.
A
B
C
D
E
F
Maximum Fuse Size
% of Primary
% of Secondary
F.L.A. (Or next
F.L.A.
size smaller.)
Max. of 125% or next larger*.
Max. 300% or next size smaller. (See
N.E.C. 430-72(c) for control circuit
transformer maximum of 500%.
Max. 167% or next size smaller.
N.E.C. MAXIMUMS
(All Fuse Types Shown)
Secondary at code max. of
125% or next standard size if
125% does not correspond to
a standard rating.
Secondary at code max. of
225% or next standard size if
225% does not correspond to
a standard rating.
Secondary Over 600V
OPTIMUM PROTECTION
Primary at code max. of 300%
or next standard size if 300%
does not correspond to a
standard rating.
Secondary at code max. of
125% or next standard size if
125% does not correspond to
a standard rating.
Secondary 600V or
Below
Secondary at code max. of
250% or next standard size if
250% does not correspond to
a standard rating.
Secondary at code max.
of 250%.
Secondary 600V or
Below
Secondary Over 600V
Secondary at code max.
of 225%.
Secondary at code max.
of 250%.
Secondary at code max.
of 250%.
Secondary Over 600V
Secondary 600V or
Below
Secondary Over 600V
(Note: Components on the secondary still need overcurrent protection.)
Rated primary current
less than 2 amps.
Transformer Impedance
Greater Than 6% But Less
Than 10%.
Primary at code max. of 300%
or next standard size if 300%
does not correspond to a
standard rating.
Transformer Impedance
Greater Than 6% But Less
Than 10%.
Transformer Impedance
Less Than or Equal to 6%.
Primary at code max.
of 300%
Transformer Impedance
Less Than or Equal to 6%.
Primary at code max. of
250% or next standard size
if 250% does not correspond
to a standard rating.
Primary at code max.
of 300%
(Note: Components on
the secondary still need
overcurrent protection.)
Primary
and
Secondary
Protection
Primary
Protection
Only
600V KRP-C_SP, LPJ_SP,
LPS-RK_SP, FNQ-R, FRS-R
Fuse
250V LPN-RK_SP, FRN-R
Fuse
2475V JCD
2750V JCX
2750/5500V JCW
5500V JCE, JCQ, JCY, JCU,
5.5 ABWNA, 5.5 AMWNA, 5.5 FFN
7200V 7.2 ABWNA, 7.2 SDLSJ, 7.2 SFLSJ
8300V JCZ, JDZ, 8.25 FFN
15500V JCN, JDN, JDM, 15.5 CAVH
17500V 17.5 CAV, 17.5 SDM
24000V 24 SDM, 24 SFM, 24 FFM
36000V 36 CAV, 36 SDQ, 36 SFQ
38000V 38 CAV
Secondary 600V or below
250V: LPN-RK_SP, FRN-R
600V: LPS-RK_SP, FRS-R
LPS-_SP, KRP-C_SP
FNQ-R_
Fuse Technology
Bussmann®
216
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Solenoids
(Coils)
Solid State Devices
(Diodes, SCR-s,
Triacs, Transistors)
Mains
Feeders
Branches
Motor
Loads
(N.E.C. 430)
Based on 1996 N.E.C.®
Supplementary
Fuses
Branch Circuit
Fuses
Short-Circuit
Protection
Only
Main, Branch &
Feeder Circuits
(601-6000 Amps)
Feeder Circuits
(600 Amps & Less)
600V & Less
Above 600V
Motor
Loads
Size at 125% or next size larger.
Size at 125% or next size smaller.
0-500 FNQ ⁄Ω¡º-30
0-250V MDL ⁄Ω¡§-8, MDA ¤Ω¡º-20, FNM ⁄Ω¡º-10, FNW 12-30, MDQ ⁄Ω¡ºº-7
0-125V MDA 25-30, FNM 12-15
0-32V MDL 9-30, FNM 20-30
251-600V LPS-RK_SP, FRS-R
0-600V LPJ_SP, LP-CC
0-250V LPN-RK_SP, FRN-R
“F”, “S”, “K” & 170M Series fuses sized up to several sizes larger than full load RMS or DC rating of device.
150% to 225% of full load current of largest motor plus 100% of full load current of all other motors plus 125% of
continuous non-motor load plus 100% of non-continuous non-motor load.
*A max. of 175% (or the next standard size if 175% does not correspond to a standard size) is allowed for all but wound rotor and
all D.C. motors.
150%* of the F.L.A. of largest motor (if there are two or more motors of same size, one is considered to be the largest)
plus the sum of all the F.L.A. for all other motors.
150%* of the F.L.A. of largest motor (if there are two or more motors of same size, one is considered to be the largest)
plus the sum of all the F.L.A. for all other motors plus 100% of non-continuous, non-motor load plus 125% of
continuous, non-motor load.
Combination
Motor Loads
and other
Loads
*150% for wound rotor and all DC motors.
Max. of 300%* of motor F.L.A. or next size larger. If this will not allow motor to start due
to higher than normal inrush currents or longer than normal acceleration times (5 sec. or
greater), fuses through 600 amps may be sized up to 400% or next size smaller.
175%* of motor F.L.A. or next size larger. If this will not allow motor to start, due to higher
than normal inrush currents or longer than normal acceleration times (5 sec. or greater),
fuse may be sized up to 225% or next size smaller.
125% of motor F.L.A. or next size larger.
100% of non-continuous load plus 125% of continuous load.
Short-Circuit
Only
Short-Circuit
Only
Backup Overload
w/ Motor Starter
& Short-Circuit
Protection
No Motor
Load
Protected by NonTime-Delay Fuses
& all Class CC Fuses
Protected by
Time-Delay Fuses
FUSE SIZED FOR:
Compare the min. melting time-current characteristics
of the fuses with the time-current characteristics of the
overload relay curve. The size fuse which is selected
should be such that short-circuit protection is provided
by the fuse and overload protection is provided by the
controller overload relays.
0-130V FWA
0-250V FWX
0-500V FWH
0-600V FWC, KAC, KBC
0-700V FWP, 170M Series, SPP
0-1000V FWJ, 170M Series, SPJ
0-600V KRP-C_SP
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
0-600V LPJ_ SP, LP-CC
0-250V LPN-RK_SP, FRN-R
0-300V JJN
251-600V LPS-RK_SP, FRS-R
301-600V JJS
0-600V JKS, LPJ_SP, KTK-R, LP-CC, LPT
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
0-600V LPJ_SP
0-250V KTN-R, NON
0-300V JJN
251-600V KTS-R, NOS
301-600V JJS
0-600V LP-CC, LPT, JKS, KTK-R
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
Fuse
2400V JCK, JCK-A, JCH
4800V JCL, JCL-A, JCG
7200V JCR, 7.2 WKMSJ
Fuse Diagnostic Chart
Bussmann®
217
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Capacitors
(N.E.C. 460)
Ballasts
Electric
Heat
(N.E.C. 424)
250% to 300%
of full load current
Protected by
Non-Time Delay
Fuses
Based on 1996 N.E.C.®
150% to 175%
of full load current
Protection recommended
as shown below, but not
required
Mercury,
Sodium, etc.
All Other
(Mercury,
Sodium, etc.)
Fluorescent
0-250V KTN-R, NON
0-300V JJN
251-600V KTS-R, NOS
0-600V JKS, KTK-R
301-600V JJS
Fuse
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
0-600V FNQ-R, LPJ_SP, LP-CC
Consult fixture manufacturer for size and type.
Consult fixture manufacturer for size and type.
Consult fixture manufacturer for size and type.
BAF
BAN
KTK
FNM
FNQ
FNW
HEB
HEX
HPC-D
HPF
HPS
HLR
GLR
GMF
GRF
BAF
BAN
KTK
FNM
FNQ
FNW
Holder
Fuse
Size at 125% or next size larger but in no case larger than 150 amperes for each subdivided load.
Size at 125% or next size larger but in no case larger than 60 amperes for each subdivided load.
Protected by
Time-Delay
Fuses
On load side of
motor running
overcurrent device
Outdoor
Indoor
Electric Boilers with
Resistance Type Immersion
Heating Elements in an ASME
Rated and Stamped Vessel.
Electric Space Heating
KTK-R
FNQ-R
LP-CC
KTQ
BBS
KTK-R
FNQ-R
LP-CC
GLQ
GMQ
Fuse
HEY
HPS-L
HPF-L
HPS-RR
HPF-RR
HLQ
Holder
Holder
HPF-EE
HPS-EE
HPF-JJ
SC 20
HPS-JJ
HPF-FF
SC 25-30
HPS-FF
SC -0-15
Fuse
Fuse
0-250V LPN-RK_SP, FRN-R, NON
0-300V JJN
0-480V SC
251-600V LPS-RK_SP, FRS-R, NOS
301-600V JJS
0-600V LPJ_SP, LP-CC, FNQ-R, JKS, KTK-R
Fuse Diagnostic Chart
Bussmann®
218
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Time-Current & Current Limitation Curves
Bussmann®
TCF & TCFH CUBEFuseTM Fuses
TCF & TCFH Time-Current Characteristic Curves–
Average Melt
219
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
KRP-C, Class L Fuses
KRP-C Current Limitation Curves
4000A
5000A
6000A
1600A
2000A
2500A
3000A
1200A
800A
KRP-C Time-Current Characteristic Curves—
Average Melt
AMPERE
RATING
300
1000,000
B
6,000A
5,000A
4,000A
3,000A
2,500A
2,000A
1,600A
1,200A
800A
601A
100
AMPERE
RATING
100,000
10
A
300,000
100,000
1,000
10,000
1
1,000
TIME IN SECONDS
10,000
PROSPECITVE SHORT-CIRCUIT CURRENT
SYMMETRICAL RMS AMPERES
200,000
100,000
1,000
.01
10,000
.1
CURRENT IN AMPERES
220
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
LPN-RK (250V) Class RK1 Fuses
AMPERE
RATING
LPN-RK_SP (250V)
300
15/100
2/10
3/10
4/10 1/2
8/10 6/10
1
1-1/4
1-6/10
2
2-1/2
3-2/10
4
5
6-1/4
8
10
12
600A
400A
200A
100A
60A
30A
Time-Current Characteristic Curves—Average Melt
1/10
300
15A
20A
Time-Current Characteristic Curves—Average Melt
100
AMPERE
RATING
LPN-RK_SP (250V)
10
TIME IN SECONDS
TIME IN SECONDS
10
1
1
.1
1,000
100
1
.1
.01
10
.1
CURRENT IN AMPERES
RMS SYMMETRICAL CURRENT IN AMPERES
10,000
1,000
100
20
.01
B
400,000
AMPERE
RATING
LPN-RK_SP (250V)
100,000
600
400
200
100
60
10,000
30
200,000
100,000
1,000
10,000
A
1,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Current Limitation Curves
RMS SYMMETRICAL CURRENTS IN AMPERES
A–B=ASYMMETRICAL AVAILABLE PEAK (2.3 X SYMM RMS AMPS)
221
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
LPS-RK (600V) Class RK1 Fuses
AMPERE
RATING
300
4
5
8
10
12
LPS-RK
(600V)
6-1/4
15/100
2/10
3/10
4/10 1/2
6/10
8/10
1
1-1/4
1-6/10
2
2-1/2
3-2/10
Time-Current Characteristic Curves—Average Melt
1/10
600A
400A
200A
100A
60A
30A
300
20A
Time-Current Characteristic Curves—Average Melt
AMPERE RATING
100
100
10
TIME IN SECONDS
1
1
1,000
1
.1
.01
.1
100
.1
10
TIME IN SECONDS
10
RMS SYMMETRICAL CURRENT IN AMPERES
AMPERE
RATING
LPS-RK
(600V)
100,000
600
400
200
100
60
10,000
30
200,000
1,000
100,000
A
10,000
20,000
B
400,000
1,000
CURRENT IN AMPERES
10,000
1,000
100
30
.01
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Current Limitation Curves
222
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
FRN-R (250V) Class RK5 Fuses
AMPERE
RATING
300
FRN-R
(250V)
15/100
2/10
3/10
4/10 1/2
8/10 6/10
1
1-1/4
1-6/10
2
2-1/2
3-2/10
4
5
6-1/4
8
10
12
Time-Current Characteristic Curves—Average Melt
1/10
600A
400A
200A
100A
60A
30A
300
15A
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
FRN-R (250V)
100
100
10
TIME IN SECONDS
TIME IN SECONDS
10
1
1
.1
1,000
100
1
.1
.01
10
.1
CURRENT IN AMPERES
AMPERE
RATING
FRN-R (250V)
100,000
600
400
200
100
60
10,000
30
100,000
10,000
1,000
1,000
A
200,000
20,000
B
400,000
INSTANTANEOUS PEAK LET-THRU CURRENT AMPERES
CURRENT IN AMPERES
10,000
1,000
20
100
Current Limitation Curves
.01
PROSPECTIVE SHORT CIRCUIT CURRENT
SYMMETRICAL RMS AMPERES
223
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
FRS-R (600V) Class RK5 Fuses
Time-Current Characteristic Curves—Average Melt
300
1/10
15/100
2/10
3/10
4/10 1/2
6/10
8/10
1
1-1/4
1-6/10
2
2-1/2
3-2/10
4
5
6-1/4
8
10
12
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
FRS-R
(600V)
100
TIME IN SECONDS
10
1
1,000
100
1
.1
.01
10
.1
CURRENT IN AMPERES
Current Limitation Curves
B
AMPERE
RATING
FRS-R (600V)
100,000
600
400
200
100
60
30
10,000
200,000
100,000
1,000
10,000
A
1,000
INSTANTANEOUS PEAK LET THRU CURRENT AMPERES
400,000
PROSPECTIVE SHORT CIRCUIT CURRENT
SYMMETRICAL RMS AMPERES
224
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
KTN-R (250V) Class RK1 Fuses
Current Limitation Curves
10
1
AMPERE
RATING
100
KTN-R
(250V)
100,000
600
400
200
100
10,000
60
30
A
1,000
100,000
KTN-R
(250V)
10,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
300
TIME IN SECONDS
B
400,000
200,000
AMPERE
RATING
1,000
600A
400A
200A
100A
60A
30A
Time-Current Characteristic Curves—Average Melt
RMS SYMMETRICAL CURRENTS IN AMPERES
A–B=ASYMMETRICAL AVAILABLE PEAK (2.3 X SYMM RMS AMPS)
.1
10,000
1,000
100
40
.01
RMS SYMMETRICAL CURRENT IN AMPERES
225
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
KTS-R (600V) Class RK1 Fuses
AMPERE
RATING
600
400
200
100
10,000
60
30
A
1,000
200,000
1
100,000
100,000
TIME IN SECONDS
10
KTS-R
(600V)
10,000
100
B
400,000
1,000
200
Current Limitation Curves
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
CURRENT IN
AMPERES
25
30
15
6
8
10
1
300
3
Time-Current Characteristic Curves—Average Melt
RMS SYMMETRICAL CURRENTS IN AMPERES
A–B=ASYMMETRICAL AVAILABLE PEAK (2.3 X SYMM RMS AMPS)
200
100
20
10
2
1
.01
300
400
500
600
700
.1
CURRENT IN AMPERES
226
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
LPJ (600V), Class J Fuses
.1
AMPERE
RATING
A
100
200,000
1
1,000
100,000
TIME IN SECONDS
10
10,000
10,000
100
600A
400A
200A
100A
60A
50A
40A
30A
20A
15A
LPJ
1,000
LPJ
B
100,000
100
AMPERE
RATING
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
600A
Current Limitation Curves
10A
15A
20A
30A
40A
50A
60A
100A
125A
200A
225A
400A
5A
3A
300
1A
Time-Current Characteristic Curves—
Average Melt
10,000
1,000
100
1
.01
10
PROSPECTIVE SHORT-CIRCUIT CURRENT
SYMMETRICAL RMS AMPS
RMS SYMMETRICAL CURRENT IN AMPERES
227
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
JJN & JJS, Class T Fuses
400A
500A
800A
200A
100A
60A
15A
30A
10A
1A
AMPERE
RATING
5A
Time-Current Characteristic Curves—Average Melt
3A
600A
400A
200A
100A
60A
30A
300
15A
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
300
JJN
(300V)
JJS
(600V)
100
100
TIME IN SECONDS
10
1
.1
1,000
1
10
.01
10,000
1
100
TIME IN SECONDS
10
CURRENT IN AMPERES
1,000
20
100
.01
10,000
.1
RMS SYMMETRICAL CURRENT IN AMPERES
Current Limitation Curves
60
30
15
100,000
200,000
10,000
A
200
1,000
1,000
RMS SYMMETRICAL CURRENTS IN AMPERES
A-B = ASYMMETRICAL AVAILABLE PEAK (2.3 x SYMM RMS AMPS)
AMPERE
RATING
400
200
100
60
30
10,000
1,000
A
200
200,000
10,000
1200
800
600
100,000
400
200
100
100,000
10,000
1200
800
600
JJS
(600V)
1,000
100,000
B
400,000
100
AMPERE
RATING
JJN
(300V)
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
B
400,000
100
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Current Limitation Curves
RMS SYMMETRICAL CURRENTS IN AMPERES
A–B=ASYMMETRICAL AVAILABLE PEAK (2.3 X SYMM RMS AMPS)
228
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
LP-CC & FNQ-R Class CC Fuses
FNQ-R-5
FNQ-R-7-1/2
FNQ-R-3
AMPERE
RATING
200
FNQ-R-1
Time-Current Characteristic Curves—Average Melt
FNQ-R-1/2
20
25
30
3
3⁄Ω™
4
4⁄Ω™
6
8
10
12
15
⁄Ω™
flꭧ
°Ω¡º
1
1⁄Ω¢
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
100
LP-CC
100
10
TIME IN SECONDS
1
200
300
400
500
100
10
1000
100
10
1
.4
CURRENT IN AMPERES
0.01
1
.01
.1
.01
1
.08
TIME IN SECONDS
10
CURRENT IN AMPERES
229
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com
Time-Current & Current Limitation Curves
Bussmann®
KTK-R, Class CC Fuses
8A
10A
15A
20A
30A
5A
3A
2A
1A
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
300
KTK-R
100
TIME IN SECONDS
10
1
400
100
1
.01
10
.1
RMS SYMMETRICAL CURRENT IN AMPERES
230
Courtesy of Steven Engineering, Inc. ! 230 Ryan Way, South San Francisco, CA 94080-6370 ! Main Office: (650) 588-9200 ! Outside Local Area: (800) 258-9200 ! www.stevenengineering.com